M GROUP THIRD SEMESTER ELECTRICAL MACHINES –1 EE 1202 1. Mention the difference between core and shell type transformers? In core types, the windings surround the core considerably and in shell type surround the winding. 2. What is the purpose of laminating the core in transformers? To reduce eddy current loss. 3. Give the EMF equation of a transformer and define each term? Emf induced in primary coil E 1 = 4.44fΦmN1 volt Emf induced in secondary coil E 2 = 4.44fΦmN2 volt Where f = frequency of A.C input. Φm = maximum value of flux in the core. N1 & N2 = No: of primary and secondary turns. 4. Does the transformer draw any current when secondary is open? Why? Yes, it (primary) will draw the current from the main supply in order to magnetize the core and to supply iron and copper losses on no load. There will not be any current in the secondary since secondary is open. 5.
Define voltage regulation of a transformer? When a transformer is loaded with a constant primary voltage, the secondary voltage decreases for lagging power factor load, and increases for leading power factor because of its internal resistance and leakage reactance. The change in secondary terminal voltage from no-load to full load expressed as a % of no-load or full load voltage is termed as regulation. %Regulation down = (0V2-V2) x 100/0V2 %Regulation up = (0V2-V2) x 100/V2
6. Full load copper loss in a transformer is 1600W. What will be the loss at ½ load? If x is the ratio of actual load to full load then copper = x2 (Full load copper loss). Here Wc = 0.52 x 1600 = 400W. 7. Define all day efficiency of a transformer? It is computed on the basis of energy consumed during a certain period, usually a day of 24 hrs. ηall day = o/p in kWh for 24 hrs.
8. Why transformers are rated in KVA? Copper loss of a transformer depends on current and iron loss on voltage. Hence total losses depend on Volt-Ampere and not on power factor. That is why ratings of transformers are in KVA and not in KW. 9. What are the typical uses of a transformer? (i) To give small boost to a distribution cable to correct for the voltage. (ii) As induction motor starters. (iii) As furnace transformers. (iv) As inter connecting transformers. (v) In control equipment for single and three phase elective locomotives. 10. What are the applications of a step up and step down transformers? Step up transformers are used in generating stations. 11. How transformers are classified according to their construction? Or Mention the difference between CORE and SHELL type transformers? Or What are the two types of core used? Compare them? The transformers are classified according to their construction • Core Type: - The windings (Primary & Secondary) surround the core. • Shell Type: - The core surrounds the windings. • Spirakore type: - Used in big transformers. 12. Explain on the material used for core construction? The core is constructed of transformers sheet steel lamination assembled to provide a continuous magnetic path with a minimum of air gap included. The steel used is of high silicon content some times heat treated to produce a high permeability and a low hysteresis loss at the usual operating flux densities. The eddy current loss is minimized by laminating the core, the lamination being insulated from each other by light coat of core-plate varnish or by an oxide layer on the surface. The thickness laminations vary from 0.35mm for a frequency of 59Hz and 0.5mm for a frequency of 25Hz. 13. When will the Bucholz relay operate in a transformer? Bucholz relay is a protective device in a transformer. If the temperature of the coil exceeds the limit, Bucholz relay operates and give alarm. 14. How does the change in frequency affect the operation of a given transformer? With a change in frequency, iron loss, copper loss, regulation, efficiency and heating and thereby the operation of the transformer is affected.
15. What is the angle by which No-load current will lag the ideal applied voltage? In an ideal transformer there are no copper loss and no core loss (i.e. loss free core) The no load current is only magnetizing current, Therefore the no load current lags behind by an angle of 90°.However the windings possess resistance and leakage reactance and therefore the no-load current lags the applied voltage slightly less than 90°. 16. List the advantages of stepped core arrangements in a transformer? i. To reduce the space effectively. ii. To obtain the reduced length of mean turn of the windings. iii. To reduce I R losses. 17. Why are breathers used in transformers? Breathers are used to entrap the atmospheric moisture and thereby not allowing it to pass on to the transformer oil, also used tp permit the oil inside the tank to expand and contract as its temperature increases and decreases also it is used to avoid the sledging of oil. Addition of 8 parts of water in 1000000 reduces the insulations of the quantity of the oil. Normally silica gel is filled in the breather having pink color. This color will change white due to continuous use, which is the indication of bad silica gel, it is normally heated and reused. 18. What is the function of transformer oil in a transformer? Now a days instead of natural mineral oil, synthetic oils known as ASKRELS (Trade name) are used. They are non-inflammable, under an electric arc; do not decompose to produce inflammable gases. This oil also possess high dielectric strength, hence it can be said that the transformer oil provides, (1) Good insulation (2) Cooling. 19. An 1100/400 V, 50 Hz single-phase transformer has 100 turns on the secondary winding. Calculate the number of turns on the primary? We know V1/V2 = k =N2/N1 Substituting 400/1100=100/N1 N1 =100/400×1100 = 275 turns. 20. What are the functions of no-load current in a transformer? No-load current produces flux and supplies iron loss and copper loss on no-load.
21. How will you transfer the quantities from one circuit to another circuit in a transformer? 1. Secondary to primary. Symbol V2i I2i R2i X
value V2/K KI2 R2/K2 X2/K2
2. Primary to secondary. symbol VLi I Li RLi XLi
value KVi I1k K2R1 K2X1
22. Define voltage regulation in a transformer? When the transformer is loaded with constant primary voltage, the secondary voltage. The secondary voltage decreases for lagging power factor load, and increases for leading. Power factor load because of its internal resistance and leakage reactance. Let V2=Secondary terminal voltage at no load E2=KE=Kv because of no load the impedance drop is negligible V2=Secondary terminal on full load. The change in secondary terminal voltage from no load to full load is =V2-E2.This change divided by V2 is known regulation down. If this change is divided by V (i.e. full load secondary terminal voltage) then it is called as regulation up. 23. Can the voltage regulation of a transformer go to negative? If so under what condition? Yes. If the load has leading power factor. 24. Distinguish between power transformer and distribution transformer? Power transformers have very high power ratings in the order of MVA.They are used in generating and receiving stations. Sophisticated controls are required. Voltage ranges will be very high. Distribution transformers are used in consumer side. Voltage levels will be medium. Power ranging will be small in order of KVA.Complicated controls are not needed. 25. What are the purpose of providing ‘taps’ in transformers and where these are provided? In order to attain the required voltage,’ taps’ are provided. Normally it will be provided at low voltage side. 26. What is a prime mover? The basic source of mechanical power, which drives the armature of the generator, is called prime mover.
27. Give the materials used in machine manufacturing? Three materials are used in machine manufacturing (1) Steel-to conduct magnetic flux (2) Copper- to conduct electric current. (3) Insulation.
28. What is MMF? Give its expression? MMF is the work done in moving a unit magnetic pole once around the magnetic circuit. MMF = N I Ampere turns. Where N =No: of turns I = Current in amperes. 29. Define magnetic field intensity? It is the MMF per unit length Amp turn/m. H=N I/L Where N=No: of turns I= Current in amperes. L=Length in meter. 30. What is reluctance? It is the property of the magnetic material to oppose the magnetic lines of flux. S=F/φ. Where F=MMF, φ=Flux., S=Reluctance in ampereturns/Wb. 31. What is stacking factor? It is the ratio between the net cross sectional area of the core to the cross section occupied by the magnetic material. 32. What is quasi-static field? It is the field pattern, which is fixed in space but field intensity at every point varies as a replica of time variation of current. 33. What is leakage flux? The flux that takes a path that is not intended for it is called leakage flux. 34. What is fringing effect? While passing through the non magnetic medium the magnetic lines of force try to bulge out because the lines of force repel each other. This is fringing effect.
35. Compare Electric and magnetic circuits? Magnetic circuit 1. Closed path of magnetic flux. 2. Flux= MMF/Reluctance. 3. Reluctance=1/aµ0 µr 4. Magnetic flux density B=ϕ/a 5. MMF drop =ϕ S 6. Magnetic field intensity H=N I/l
Electrical circuit Closed path for electric current. Current=EMF/Resistance. Resistance R=ρl/a. Current density J=I/a Voltage drop=I R. Electric field intensity E=V/d
36. How is the direction of induced emf determined? Direction of induced emf is determined by using (i) Right Hand Gripping Rule. (ii) Right Hand Cork Screw Rule. 37. State Lenz’s law? Any induced emf will circulate a current in such a direction as to oppose the cause producing it. e = -N dφ/dt 38. What is self-inductance? The emf induced in a coil due to change of flux in the same coil is known as self-inductance. 39. Give the expression for magnetizing force? F = B I l sin θ Newton, Where I-Current carried by the conductor. l-Effective length of the conductor. B-Flux density in Wb/m2 40. Derive an equation for energy density for a magnetic circuit? Energy stored in the magnetic circuit = ½ L I2 L = N2 a µ0 µr / l. Energy density = ½ I2 N2 a µ0 µr / l. = ½ B2 H2 a µ0 µr. (. . . H = N I / l) = ½ B2 l a / µ0 µr = ½ B2 a2 / S = ½ I φ2 / R Joules
41. What are the factors on which hysteresis loss depends? The hysteresis loss depends on the magnetic flux density, frequency and the volume of the material.
42. What is core loss? What is its significance in electric machines? When a magnetic material under goes cyclic magnetization, two kinds of power losses occur on it. Hysteresis and eddy current loss, which together are known as core loss. It is important in determining heating, temperature rise, rating and efficiency of transformers, machines and other a.c. run magnetic devices. 43. What is eddy current loss? When a magnetic core carries a time varying flux voltages are induced in all possible paths enclosing flux. Result is the production of circulating current in the core. These induced currents do no useful works are known as eddy current and have power loss known as eddy current loss. 44. How are hysteresis and eddy current losses minimized? Hysteresis loss can be minimized by selecting materials for core such as silicon steel & steel alloys with low hysteresis co-efficient and electrical resistivity. Eddy current losses are minimized by laminating the core. 45. What is coupling co-efficient? k = M / √ L1 L2 Coupling co-efficient is defined as the ratio of mutual inductance to the square root of the product of two self-inductances.
46. Write an expression for energy stored in magnetic field. Wf =∫b1b2Hcdb, Where Hc =Magnetic field intensity in the core. 47. How does electro mechanical energy conversion occurs? It occurs through the medium of the magnetic stored energy. 48. Write the equation for energy transfer. Energy input from electric sources =mechanical energy output + increase in energy stored in magnetic field + Energy converted into heat For motors: electric and mechanical energy terms have positive values For generators: they have negative values. 49. What is the energy conversion medium in a singly excited magnetic field system? The magnetic circuits have air gap between stationary and moving members in which considerable energy is stored in the magnetic field. This field acts as the energy conversion medium and its energy is the reservoir between electric and mechanical system.
50. What is the basic feature of electro magnetic energy conversion devices? They contain air gaps in their magnetic circuits in their moving parts. 51. Why does the energy storage in a magnetic material occur mainly in the air gap? The reluctance of the air gap is much larger than the magnetic material. Hence the predominant energy storage occurs in the air gap and the properties of the air gap are determined by the dimensions of the air gap. 52. What is multiple excited magnetic field system? If the electro mechanical devices have more than one set of exciting system, it is called multiple excited magnetic field system. 53. Give the relation between energy and co-energy for linear system. They are numerically equal in linear system. 54. Why is a magnetic energy storage system conservative? Since it is loss less. 55. How is voltage generated in rotating machines? In rotating machines voltage is generated in windings or group of coils by rotating them through a magnetic field or by mechanically rotating a magnetic field past the winding or by designing the magnetic circuit so that the reluctance varies with rotation of the rotor. 56. Why are magnetic coils wound on iron cores? To maximize the coupling between the coils, to increase the magnetic energy density associated with the electro mechanical interaction and to shape and distribute the magnetic field according to the machine design. 57. Why salient pole construction is a characteristic of hydroelectric generators? Because hydraulic turbines operate at relatively low speeds and large number of poles are required to produce the required frequency. 58. What are distributed windings? Windings, which are spread over a number of slots around the air gap periphery. 59. Write the equation for total co-energy density? Wfld=average co-energy density x volume of air gap. =µο/4[Fsr/g]2ΠDlg Where D =average diameter of air gap l = axial length of air gap. µο =Permeability of free space. 60. What is the expression for torque in terms of the resultant mmf wave Fsr? T=-P/2.Π/2.µοDl/g Fr Fsr sinδ
61. What is torque proportional to? Torque is proportional to the interacting fields and to the sine of the electrical space angle between their magnetic axes. 62. How will you find the direction of emf using Fleming’s right hand rule? The thumb, the forefinger and the middle finger of the right hand are held so that these fingers are mutually perpendicular then Forefinger –field Thumb-motion Middle finger-I, current. 63. How do you find the direction of force produced using Fleming’s left hand rule? The thumb, the forefinger and the middle finger of the left hand are held so that these fingers are mutually perpendicular then Forefinger –field Thumb-motion (due to force) Middle finger-I, current. 64. Which type of dc armature winding requires equalizer ring? Lap winding. 65. How is armature windings classified based on the placement of the coil inside the armature slots? Single layer and double layer windings. 66. Write down the emf equation for dc generator? E=(φNZ/60)(P/A) V. Where P =no. of poles Z =total no. of conductors A =no. of parallel plates φ =Flux per pole N =speed in rpm. 67.Why the armature core in the DC machines is constructed with laminated steel s sheets instead of solid steel sheets? Steel sheets offer low reluctance path for the magnetic field, laminated sheets reduce eddy current loss. 68.Why is commutator employed in DC machines? • Conduct electricity between armature and fixed brushes • Converts alternating emf into unidirectional emf and vice versa.
69. What is meant by selective commutation? The use more than one pair of brushes in wave winding does not divide the armature coil sides into more than two parallel paths, but current collected from the armature is divided between the brushes of like polarity. In case of slight differences in contact resistance the current collected by individual brushes may be different and is called selective commutation. 70. Distinguish between shunt and series field coil construction? Shunt field coils are wound with wires of small cross section and have more no of turns. Series field coils are wound with wires of larger cross section and have less no of turns. 71. How does a dc motor differ from dcgenerator in construction? Generators are normally placed in closed room, accessible only to skilled operators therefore on ventilation point of view they may be constructed with large opening in the frame. Motors on the other hand, have to be installed right in the place of use which may havedust, dampness, inflammable gases, chemical fumes, etc. To protect the motor against these elements the motor frames are made either partly closed or totally closed or flame proof etc. 72. How will you change the direction of rotation of a D.C motor? Either the direction of the main field or the direction of current through the armature conductors is to be reversed. 73. What is back EMF in dc motors? As the motor armature rotates, the system of conductor comes across alternate North and South Pole Magnetic fields causing an EMF induced in the conductors .The direction of the EMF induced is in the direction opposite to the current. As this EMF always opposes the flow of current in Motor operation it is called as Back EMF. 74 Under what condition the mechanical power developed in a D.C motor will be maximum? Condition for mechanical power developed to be maximum is E b=Ua/2 Or Ia = Ua/2Ra 75. What is the function of no-volt release coil provided in D.c Motor starter? As long as the supply is on healthy condition the current through the NVR coil produce enough magnetic force and retain the starter handle is in the On position against spring force.When the supply voltage fails or become lower than a prescribed value the electromagnet may not have enough force and the handle will come back to OFF position due to spring force automatically. Thus a no-voltage or under voltage protection is given to the motor.
76. Name the two types of automatic starters used in D.C motors? i. Back emf type starter. ii. Time delay type starter. 77. Enumerate the factors on which the speed of a motor depends? N =1/Ce (Ua - Ia Rm) ϕ The speed of the motor depends on the three factors (1) Flux in the air gap. (2) Resistance of the armature circuit. (3) Voltage applied in the armature. 78.Draw the N Vs Eb Characteristics of a D.C motor for different field currents? Nn If
Ehn 79 List the different methods of speed control employed in D.C motors? i. Field diverter method ii. Re grouping of field coils. iii. Tapped field control. iv. Armature resistance control. v. Armature voltage control for single motor. vi. Series parallel control for multiple identical motors. 80. Name the different methods of braking of D.C motors? (1) Dynamic breaking. (2) Regenerative braking (3) Counter current breaking or Plugging.
81. Under what circumstances does a D.c Shunt generator fails to build up? (1) Absence of residual flux. (2) Initial flux set up by the field winding may be in opposite direction to that of the residual flux. (3) Shunt field circuit resistance may be higher than uts critical field resistance.
82. How can one differentiate between long shunt compound generator and short shunt compound generator? In short shunt compound generator the shunt field ckt is shorter i.e across the armature terminals. In a long shunt compound generator the shunt field ckt is connected across the load Terminals. 83. Why is the EMF not zero when the field current is reduced to 0 in a d.c. generator? Even after the field current/magnetizing force is reduced to zero the machine is left out with some flux as residue. 84. Define the term critical speed in D.C shunt generator? Critical speed is defined as the speed at which the generator is to be driven to cause selfexcited generator to build up its emf for the given field circuit resistance. 85. On what occasions dc generators may not have residual flux? i. The generator may be put for its first operation after its construction. ii. In previous operation the generator would have been fully demagnetized. 86. What are the conditions to be fulfilled for a dc shunt generator to build up an emf? i. The generator should have residual flux. ii. The field winding should be connected in such a manner that the flux set up by the field winding should be in the same direction as that of residual flux. iii. The field circuit resistance should be less than critical field resistance. iv. The load circuit resistance should be above its critical load resistance. 87. How the critical field resistance of a dc shunt generator is estimated from its OCC? Critical field resistance can be obtained from OCC by drawing a straight line passing through the origin and tangent to the initial straightline portion of OCC. The slope of this line gives the value of critical field resistance for the given speed at which OCC is obtained.
88.Define the term armature reaction in dc machines. The interaction between the flux setup by the current carrying armature conductors with the main field flux is defined as armature reaction. 89. What are the two unwanted effects of armature reaction? i. Cross magnetizing effect /Distorting effect. ii. Demagnetizing effect. 90. Differentiate between geometric neutral axis (GNA) and magnetic neutral axis (MNA)? GNA is the axis, which is situated geometrically or physically in the midway between adjacent main poles. MNA is the axis, which passes through the zero crossing of the resultant magnetic field waveform in the air gap. 91. In which part of the dc machine is the compensating winding situated? In the slots provide in the main pole phases. 92. What are the various types of commutation? iii. Linear commutation. iv. Sinusoidal commutation. 93. Name the two methods of improving commutation? v. Emf commutation. vi. Resistance commutation. 94. What is the reactance emf in the dc machines? The self-induced emf in the coil undergoing commutation, which opposes the reversal of current, is known as reactance emf. 95. Define the term commutation in dc machines. The changes that take place in the winding elements during the period of short circuit by a brush is called commutation. 96. How and why compensating winding in dc machine excited? As the compensation required is proportional to the armature current the compensating winding is excited by the armature current. 97. How is the interpole winding in dc machine excited? Inter pole winding is connected in series with the armature circuit and is excited the armature current. 98. To what polarity are the interpoles excited in dc generators? The polarity of the interpoles must be that of the next main pole along the direction of the rotation in the case of the generator.
99. To what polarity are the interpoles excited in dc motors? For motor operation the polarity of the interpoles must be that of the previous main pole along the direction of rotation. 100.Why are carbon brushes preferred for dc machines? The high contact resistance carbon brushes help the current the coil undergoing commutation to attain its full value in the reverse direction at the end of commutation. The carbon brushes also lubricate and give less wear and tear on commutator surface.